In a mobile communication system like a widespread mobile telephone system today, the entire service area is usually divided into rather small areas called cells to provide services. Such a system comprises as shown in FIG. 1, for example, a plurality of base stations 102 for covering divided cells 106, and mobile stations 104 for conducting communication with the base stations 102 by establishing radio channels with the base stations.
The radio waves transmitted from the base stations 102 or mobile stations 104 at certain transmission power travel through the space with some attenuation and arrive at a receiving point. The attenuation the radio waves undergo usually increases with the distance between the transmitting site and the receiving site. Besides, the propagation loss varies greatly due to the surrounding geography and conditions of objects because the radio waves undergo blocking, reflection and diffraction by large buildings, mountains or hills. On the other hand, the receiving side requires received power higher than a certain level to receive and demodulate signals at a prescribed level of quality. Accordingly, it is very important for area design in mobile communication systems to cover the service area efficiently by utilizing limited transmission power.
To achieve such design, a method is often used which estimates radio wave propagation conditions in the service areas by simulating the radio wave propagation, from the specifications of the base stations and mobile stations and geographic data using a computer. Such a method is described, for example, in “Cell Design System in Mobile Communication” by Fujii, Asakura and Yamazaki, NTT DoCoMo Technical Journal Vol. 1, No. 4, pp. 28-34, 1995-01, or in “Total Support System for Station Establishment Design” by Ohmatuzawa and Yamashita, NTT DoCoMo Technical Journal Vol. 4, No. 1, pp. 28-31, 1996-04. These methods divide the cells into smaller subdivisions, store altitude data, geographic data and communication traffic data of individual subdivisions, and calculate the signal-to-interference ratio (SIR) at each receiving site or traffic for each base station. These methods employ as a multiple access scheme, frequency division multiple access (FDMA), or time division multiple access (TDMA).
On the other hand, as for code division multiple access (CDMA), not only the propagation conditions that vary the performance of the FDMA and TDMA, but also the communication traffic and its temporal variations have great effect on the performance. Japanese Patent Application laid-open No. 8-191481 (1996), “Call Admission Control Method and Apparatus”, discloses a method of deciding the admission of a new call on the basis of an estimate of interference at a base station, in which it is emphasized that the interference is an important factor of deciding the performance of uplink channels in a CDMA system.
Furthermore, International Publication No. WO98/30057 “Call Admission Control Method and Mobile Station in CDMA Mobile Communication System” discloses a method for a mobile station to make a call admission decision by transmitting information on the uplink interference and downlink transmission power from the base station to mobile stations, in which it is emphasized that the total transmission power of the base station is an important factor of deciding the performance for the downlink. Such a scheme that carries out the area design with considering the communication traffic and its time variations is new to the CDMA system.
The foregoing conventional schemes have a great problem of being inapplicable to a CDMA system without change because interference caused by communications conducted by neighboring base stations is not counted as the traffic.
In addition, although the interference power from the same radio channel reused in distant sites greatly degrades the performance in the FDMA or TDMA system, since it is not counted as the traffic, it presents a problem of hindering accurate performance calculation.
Furthermore, since the conventional scheme does not consider the total downlink transmission power that serves as an important index in the CDMA system, it has a great problem of being inapplicable to the CDMA system without change.
Moreover, although the total transmission power has a great effect on the performance in the FDMA or TDMA system with a configuration of amplifying signals in common which are transmitted using multiple channels, there is no calculation method applicable to such a configuration, which presents a problem of being unable to calculate its performance.
In a communication system in which many users share a limited number of communication channels, there arises an occasion on which no communication channel is assignable to a user.
In a general communication system such as a fixed telephone system or mobile telephone system, many users share communication resources. For example, consider an office telephone system shared by ten employees. The probability that the ten employees conduct telephone conversations at the same time is very small, nearly zero. Thus, the number of circuits required in the office can be less than ten, and the employees share the limited number of channels, use them when necessary, and release them after their conversations so that other employees can use them. However, it sometimes takes place that no channel is available because all the channels are busy. In such a case, many of the present communication systems reject a new call, resulting in a call loss. It is preferable that the number of channels be as small as possible from an economical point of view. An excessively small number of channels, however, will increase the call loss and complaints of the employees, or hinder smooth processing of jobs. To satisfy such conflicting requirements, the Erlang B formula (see, Leonard Kleinrock, “Queueing Systems Volume I: Theory”, John Wiley & Sons, pp. 105-106, 1975, for example) is used to implement a sufficiently small blocking probability of about 1% to 3%, for example, in designing the number of channels.
Such consideration about the office telephone system is also applicable to the fixed telephone system and the mobile communication system, as well. In particular, in the mobile communication system, communications between the base stations and mobile stations are established using radio transmission, and the resources the communications use are radio channels. Since the frequency band available for the mobile communication system is generally limited, the resource sharing becomes a more serious issue than in the fixed telephone network that uses wired circuits for information transmission. As radio channel access schemes the mobile communication usually utilizes, there are frequency division multiple access (FDMA), time division multiple access (TDMA) or code division multiple access (CDMA).
In the FDMA or TDMA system, the number of radio channels to be assigned to the stations can be designed using the Erlang B formula as in the conventional system because the radio frequencies available are assigned to the base stations in advance. In the CDMA system, however, since the base stations share the same radio frequency band, the conventional method is inapplicable.
The International Publication No. WO98/30057 “Call Admission Control Method and Mobile Station in CDMA Mobile Communication System” discloses a method of making a call admission decision on the basis of the uplink interference power observed by the base station and transmission power of the base station. It, however, only describes the method of making a decision as to whether the call is acceptable or not, and cannot obtain the blocking probability from the traffic applied to the system.
On the other hand, there is an example that formulates the relationship between the applied traffic and the blocking probability in the CDMA system. For example, a paper by A. M. Viterbi and A. J. Viterbi, “Erlang capacity of a power controlled CDMA system”, IEEE J. Select. Areas Commun., Vol. 11, pp. 892-900, August 1993, discloses a method of calculating the mean and variance of the interference power observed by the base station from applied traffic, and simply calculating the blocking probability on the assumption that the interference power is normally distributed. The paper calculates the blocking probability Pblocking by the following formula.
                              P          blocking                ≈                  Q          ⁡                      [                                          A                -                                  E                  ⁡                                      (                                          Z                      ′                                        )                                                                                                Var                  ⁡                                      (                                          Z                      ′                                        )                                                                        ]                                              (        1        )            where E{Z′) is the mean of the normalized interference, Var{Z′) is the variance of the normalized interference, both of which are expressed as a function of the applied traffic. On the other hand, A is a threshold value of the normalized interference, and Q(x) is defined by the following equation.
                              Q          ⁡                      (            x            )                          =                              ∫            x            ∞                    ⁢                                    1                                                2                  ⁢                  π                                                      ⁢                                                  ⁢                          ⅇ                                                -                                      t                    2                                                  /                2                                      ⁢                          ⅆ              t                                                          (        2        )            
Expression (1) corresponds to calculating the probability that the normalized interference, which is assumed to have the normal distribution, exceeds the threshold value A.
In practice, however, the interference power reduces when the call loss is present, and accurate calculation of the blocking probability is impossible without considering the reduction in the interference power due to the call loss.
FIG. 14 is a block diagram illustrating the calculation of the blocking probability by the conventional technique (disclosed by the foregoing paper). The method described in the paper calculates the blocking probability without considering the reduction in the interference power due to the call loss. Thus, it has a serious problem of being unable to calculate the blocking probability accurately.